Process of chemical nickel plating of aluminum and its alloys and baths therefor



Uniifid States Patent O PROCESS OF CHEMICAL NICKEL PLATING OF ALUMINUM AND ITS ALLOYS AND BATHS THEREFOR Raymond R. Re'schan, East-Chicago, andAbraham Krieg,=

Gary, Ind., assignors-toGeueral American Transportation Corporation, Chicago, 111., a corporation of New York No Drawing. Application September 16,1952, Serial-N0. 309,939

Claims. (Cl. 117-50).

The present invention relates to improved processes of.

chemical nickel platingzof aluminum andits alloys emfi ploying baths. of the nickel cation-hypophosphite anion type, and to improved baths employed insuch processes.

Thechemical'nickel plating of any catalytic material,v

including aluminum and its alloys, employing an aqueous acid bath of the. nickelcation-hypophosphite anion type is based upon the catalytic reduction. of nickel cations to-metallic nickel. and the corresponding oxidation of hypophosphite anions. to phosphiteanions with the evolution of hydrogen gas at thecatal-ytic surface. Thereactions. take placewhenthe body of catalytic mate rial is immersed inthe bath, and the exterior surface of, the body of catalytic material is coated with nickel, The chemical nickel plating process is autocatalytic since both the original surface of the body being plated and the-- nickel metal that is deposited .on the surface thereof are catalytic;: and thereduction of the nickel cations t to metallic nickel inthe bath proceeds until all of the nickel cations.have been reducedto metallic,nickel,, in the presence ofan excess of hypophosphite anions, or until all;ofthe hypophosphite anionshave been oxidized tophosphite-anions, in the presence of an excess of nickel cations. Actually the reactions are slowed down rather rapidly as time proceeds because the anions, as contrasted with the cations, of the nickel salt that is dissolved in the bath combine with the-hydrogen cations to form an acid, which, in turn, lowersthepHofthe bath, tending. to' dissolve the nickel deposit. Also the reducing power of the hypophosphite. anionszis decreased as the pH -value '.of the bath decreases;

blackr precipitate that comprises .a random chemical: non-,catalytic'reduction of"the:n1ckeli:cations; which for,

mation oflthe blacldprecipitate is alsov favored by: a high absolute concentration of :hypophosphite anions: in;

the bath. Of course, this formationt of black: precipitate? comprises a vdecornpositionzofthe 'bath,v and is further objectionable in .thatvit .icauses Lth'ewnickelideposit' to be coarse andrough.

For the dual purposes of increasing the stability-of the bath (preventing the: formation: of the "black:'precipitate" mentioned) j and of increasing the normalu'plating rate of .thebath, variousbathsof the present type have been suggested employing-different additives that-serve I either as bufiersor as exaltants. Forexample,rin U. S.

Patent No. 2,532,283, grantedon December-i5, 1950, to Abner Brenner and Grace Riddell; there are shown f two'chemical nickel plating: baths ofJthe-aqueous acid nickel,:cation-hypophosphite anion. type :that respectively. contain as butter additives-"sodium i.acetateand-.sodium':

citrate; and in the copendiiigrapplication. ofiGregoire Gutzeit and Ernest J. Ramirez, Serial No. 204,424, filed January 4, 1951, now.Patent No.. 2,658,842, granted No--- vember 10,1953, there are disclosed anumber. of. chemi-I. cal nickel plating baths of the aqueous acid nickelcationhypophosphite .anion .type that contain as exaltant additivessvarious salts of simple short chain saturatedali phatic dicarboxylic acids, including sodium succinate.

While the above-mentioned -'stabilized-'baths of the nickel .cation-hypophosphite .anion: type. arc'iwell-suited;

Moreover, there is a tendency :for theub'ath, asr the :pI-L value thereof: is de-w creased, to.v become 'unstable': With :the formationv of a to .the.chemical'nickelr fplatingpf such? catalytic imateria'ls as copper, chromium, cobalt, iron, nickel and palladium}.

they; are notzrideally 1 suited- :to 1 the,- :chemical; .nickel plat;

ice

2. ingof aluminum" andits-alloys, since aluminum and its? alloys-are attacked by the bath due to its low pH (acid corrosion), and since the reactions-involved proceedto'o' in a decrease of'tlieeffectivereducing'power of the hypo phosphite anions: Inany'case,.a bath of the character noted containing the nitrate additive mentioned results inthe chemical nickel platingof aluminum and its all f at a=satisfactory* rate and without the formation of' blackprecipitate in" thebath, whereby'the nic'kelde posit-is' 'smo'othand-bright; Also, it'lfas b'e'enj discover d that the platingrate of such a stabilized bath-ofthe' character" described" may be i greatly increased, Without inducinginstability'therein, by employing as, an addi tionaha'dditive; a'srnallamount of a solublefiuoride', such' as sodiumfiuoride', potassium fluoride', ct'c.

bath ofthe nitrateand the fluoride additives brings about inthe chemical nickel plating of aluminum'jan'd'its,

alloysa great stability'of the'bath andahigli nickel} plating rate; rendering" such baths ideally suited to coin'-' m'ercrl chemical nickel plating of bodies formed es'se'fr tial-ly. ofalumi'num or aluminum alloys;

Itgis, therefore, the primar'ywbjectof the present in-- vention to provide an "improved chemical process of plat ing With nickel, aluminum and its alloys, in vvhi'ch"the' reactions involved are" carried out more 'cfiic'ie'ntly" and perfectly. 'than"heretofore; thereby rendering the process, more dcsirable from a commercial standpoint,

Another object of the invention is to provide "an'im' proved processof'the character described that employs a bath of the nickel"'cation-hypophosphite type contain; ing as anadditive nitrat'e'anio'ns.

Anotherobject ofthe" invention is to provide an im-' a bath of the nickel cation-hypophosphite anioniype' containing as-additives both nitrate anionsand fluoride anions,

A further object' of the invention is to provide improved process of the character described that employs prove'dj aqueous acidbaths of the character described thatare' particularly Well-suited to the chemical nickel plating of aluminum and its alloysl' These and other objects and advantages of the present" invention'will' be understood from the foregoing and followingjdescription.

In accordance with the process of the present invent1on,;the article formed essentially of aluminum or "an aluminum 'alloythat is to be nickel plated is first prope rly prepared by mechanically cleaning, de'g'reasingland l ght pickling, This preliminary preparation of the article is very'important si'nceit is essential to good adhes onof the nickel deposit. The pretreatment of the article may be fundamentally inaccordance with standardjpract'lce in electroplating processes; and-the followmg pretreatment is recommended as it has been found to be highly Satisfactory;

First, the article is foreign matter, and then subjected-to a standard vapor degreasing step. Next, the article is pickled for about- 30 seconds in anaqueoushydrochloric acid solution, abo'ut lzl at approximately 60 C., or somewhat more concentrated at a lower temperature The article is then rinsed, with Water (preferably hot), and then immersed for about-30 seconds'in an aqueous solution of approximately 3 parts concentrated nitric acid (38% HNOa) and '1 part concentrated aqueous hydrofluoric ac'id.' article is 'again rinsed- With Water (preferably hot), and

immediately immersedin a hot plating bathof the nickel cation-hypophosphite anion type of the character noted-' and of a composition more fullydes'cribedhereinafter at t N v- 1954.

I The" combination in the" nickel cation-hypophosphite anion su'itablycleaned of anyscale orf Finally, the

the bath being maintained at a temperature 98i1 C. under standard conditions of pressure.

Almost immediately hydrogen bubbles are formed on the catalytic surface of the article and escape in a steady stream from the bath, while the surface of the article s slowly coated with metallic nickel. Actually the coatmg or deposit on the surface of the article constitutes an alloy of nickel and phosphorus containing about 3% to 8% phosphorus, by weight, but has the appearance of metallic nickel and is considered to be a metallic nickel deposit for the purpose of the disclosure of the present invention. In general, best chemical nickel plating results are obtained when the ratio (V/A) between the volume of the bath (cm?) and the surface area of the article being plated (cm?) is below 10.

With respect to the composition of the bath, it essentially comprises an aqueous acid solution containing nickel cations, hypophosphite anions and nitrate anions, and may be formed by dissolving in a suitable acid water solution, a soluble nickel salt, a soluble hypophosphite, and a soluble nitrate. For example, the nickel cations may be derived from nickel chloride (21 commercial grade), the hypophosphite anions may be derived from sodium, potassium, calcium, magnesium, barium, etc., hypophosphitegor various combinations thereof; and the nitrate anions may be derived from sodium, potassium, nickel, etc., nitrates, or various combinations thereof. In passing, it is noted that certain alkaline cations that may be thus introduced into the bath appear to retard in the process the rate of nickel deposition with respect to other cations; for example, barium cations appear to retard the rate of nickel deposition with respect to sodium and potassium cations. Specifically, a suitable bath may be formed in an exceedingly simple manner by dissolving in a hydrochloric acid water solution commercial nickel chloride, sodium hypophosphite and sodium nitrate. The desired pH of the bath may be established by the introduction thereinto of additional hydrochloric acid and may be appropriately adjusted by the addition thereto of a weak alkali, preferably sodium bicarbonate. Also, it will be understood that the nitrate anions and some of the hydrogen cations may be derived by introducing nitric acid, instead of sodium nitrate, into the bath, although ordinarily, it is easier to employ sodium nitrate for the primary purpose noted.

Also, as explained more fully hereinafter, the nickel plating rate upon aluminum and its alloys of a bath of the character above described may be materially increased by providing therein fluoride anions; which may be readily accomplished by adding a soluble fluoride to the bath in the preparation thereof. Specifically, the fluoride anions may be derived from sodium, potassium, etc., fluorides, or various combinations thereof. Also, it will be understood that the fluoride anions and some of the hydrogen cations may be derived by introducing hydrofluoric acid, instead of sodium fluoride, into the bath; although ordinarily it is easier to employ sodium fluoride for the primary purpose noted.

The terms cations, anions and ions as employed herein include the total quantity of the corresponding elements that are present in the bath; i. e., both undissociated and dissociated material. In other words, 100% dissociation is assumed when the terms noted are used in connection with molar ratios and concentrations in the various baths.

For the purpose of demonstrating the stabilizing effect of nitrate anions upon chemical nickel plating baths of the aqueous acid nickel cation-hypophosphite anion type, a series of comparative nickel plating tests were conducted employing baths of the character of those set forth in the previously mentioned Brenner and Riddell patent.

Specifically, a first series of chemical nickel plating baths were prepared that contained the recommended ingredients, parts by weight:

Nickel chloride (NiCl2.6HzO) parts 3 Sodium hypophosphite (NaH2PO2l-I2O) parts- 1 Sodium citrate (2 (NaaCsHsOr) .l lHzO) parts 1 pH 5.30-5.35 Temperature C 96 A second series of chemical nickel plating baths were prepared that contained the recommended ingredients, parts by weight:

Nickel chloride (NiCl2.6H2O) parts I 3 Sodium hypophosphite (NaHzPOaHzO) Also, there were prepared a third series of chemical nickel plating baths (identical to the first series set forth above), but also containing 0.35 mole/liter of nitrate anions derived from sodium nitrate; and a fourth series of chemical nickel plating baths (identical to the second series set forth above), but also containing 0.35 mole/liter of nitrate ions derived from sodium nitrate.

Employing these four series of chemical nickel plating baths, plating tests were conducted employing samples formed of aluminum and aluminum alloys that were properly pretreated, in the manner previously described. The series of samples comprised the aluminum and aluminum alloys as follows: 2-8, 25S, 52-8 and 616.

In each case, employing the first and second series of chemical nickel plating baths above set forth, a violent reaction took place at the catalytic surface of the sample, and the bath was decomposed within a few minutes (2 to 3 minutes). in other words, each of the chemical nickel plating baths in the first and second series above described was decomposed after a few minutes by the random reduction of the nickel cations to metallic nickel with the formation of black precipitate, whereby the bath was rapidly exhausted of its nickel cation content, with no or very little and spotty nickel plating. In each case, employing the third and fourth series of chemical nickel plating baths above set forth, a gentle reaction took place at the catalytic surface of the sample; the bath was not decomposed by the formation of black precipitate; and the nickel plating produced upon the sample was good. However, the plating rate was very slow, less than 0.0001" per hour.

These sets of comparative tests clearly demonstrate that the addition of the soluble nitrate is effective in the preventing of decomposition of the bath by random reduction of the nickel cations therein and the consequent formation of black precipitate when the bath is employed in the nickel plating of aluminum and its alloys. It is postulated that the addition of the soluble nitrate probably results in inhibition of the catalytic surface of the sample, formed essentially of aluminum or of aluminum alloys, resulting in an effective decrease of the reducing activity of the hypophosphite anions.

Still other comparative tests of the character set forth above were conducted for the purpose of determining the proper content of the nitrate anions, and it was discovered that a large excess of nitrate anions unduly slows down the reaction, whereas an insufficient amount of nitrate anions is not adequate to prevent the random reduction of the nickel cations with the consequent formation of the black precipitate (metallic nickel sponge). Specifically, it was discovered in a bath of the character specified that the slow-down of the reactions was unduly objectionable when the content of the nitrate anions was in excess of about 0.50 mole/liter, and that the formation of the black precipitate was not inhibited when the content of the nitrate anions was below about 0.15 mole/liter.

The stability of a fast chemical nickel plating bath of the character of that disclosed in the previously-mentioned Gutzeit and Ramirez application is also enhanced in a similar manner by the addition of nitrate anions thereto; which increased stability of the chemical nickel plating bath results in a slow-down of the reactions as indicated by the following test:

A 24 aluminum sample with a surface area of 20 cm. was immersed in 50 cc. of an aqueous acid plating solution (C/A-2.5), heated to and maintained at 98 C., that contained the following ingredients:

The initial pH of this bath was 4.6; and the weight gain of the sample in 10 minutes was 0.0775 gm., corresponding to a nickel plating rate in gm/cmF/min. of 3.87Xl0- In order to increase the nickel plating rate still further.

ner disclosed in the copendi1-1g.,.application of Gregorie Gutzeit Serial No. 283,825, filed April 23, 1952,. ,Specifically, the above-mentioned bath was modified by the addition thereto of 0.02 mole/liter of .fluoride ,anions derived from sodium fluoride; .anda .testidentical to that described above was repeated. In this test, the weight gain of-the 2% aluminum sample was 0.0815 gm. :in 10 mins., corresponding to the rate of nickel plating jin gm./cm. /min. of 4.06 10 Afurther series of nickel plating "tests were conducted with the view of determining thebest perimeters of the chemical nickelplating bath with regard to the -.c ntent of nitrate anions and fluoride anions, with reference to the different aluminum alloys, and it was discovered that in each case best chemical nickel plating was obtained upon the various aluminum and aluminum alloy samples when the temperature of the bath was maintained at approximately 98 C., and the pH of the bath was adjusted within the relatively narrow range 4.5 to 4.7 employing hydrochloric or lactic acid, and that in general the composition of the bath should be:

Mole/liter Nickel cations 0.09 Hypophosphite anions 0.225 Succinate anions 0.06 Nitrate anions 0.18 to 0.35 Fluoride anions 0.01 to 0.04

Mole/liter of N03- 2-S aluminum 0.18 to 0.22 52S aluminum alloy 0.28 24-S aluminum alloy 0.35 61-S aluminum alloy 0.35

Further, it was discovered that the concentration of fluoride anions in the baths should range from about 0.01 to 0.04 mole/liter; that amounts of fluoride anions in the bath in excess of about 0.04 mole/liter are practically without additional elfect upon the nickel plating rate, although they are not objectionable; and that amounts of fluorine anions less than 0.01 mole/liter are without any practical effect upon the nickel plating rate.

In carrying out the foregoing approximately determined perimeters, a number of samples of 2-8 aluminum having an area of 20 cm. were pretreated, as previously described, and plated at 98 C. in an aqueous acid bath of the character mentioned containing the following ingredients:

Mole/liter Nickel cations 0.09 I-Iypophosphite anions 0.225 Succinate anions 0.06 Nitrate anions 0.35 Fluoride anions 0.02

These baths were prepared employing commercial nickel chloride, sodium hypophosphite, sodium succinate, sodium nitrate and sodium fluoride. In four tests employing the 2-8 aluminum samples mentioned, the following results were obtained:

Initial pH 4. 60 4. 60 4. 60 Final pH 3. 13 3.12 3. 00 2. 04 Plating time (mi 10 0 120 Weight gains (gm.) 0. 0906 0. 0881 0. 1800 0. 1761 Plating rate, gmJcmJfmin. X 10 4. 33 4. 40

6 brings about :the plating 40f bright, smooth :nickel .deposits upon articlesnformed lofaluminum and the .various alloys ,of aluminum, at .;a uery satisfactory plating rate and {the nickel .deposits are intimately and tenaciously bonded to the articles provided 'the'articles are properly pretreated, as previously described. However, in the absence of properpretreatment .of the articles, poor.adhesion of the nickel coatings thereon :results.

Inviewof theforegoing, it :is apparent that there has been provided an improved process of chemically plating articles formed essentially ;of (aluminum and aluminum alloys, with nickel, and :an improved bath of the nickel cations-hypophosphite,anion type that is expressly useful in the plating. of aluminum .and its ,alloys, wherein the bath is ofa ,stablecomposition and the nickel plating rate thereof is greatly increased with respect to prior processes of this character, by virtue of the incorporation in the bath of the nitrate anions or the combination of the nitrate anions and the fluoride anions.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. The process of chemically plating with nickel a body essentially formed of a material selected from the group consisting of aluminum and its alloys, which comprises contacting said body with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate, wherein the absolute concentration of nitrate anions in said bath expressed in mole/liter is between 0.15 and 0.50.

2. The process set forth in claim 1, wherein the initial pH of said bath is in the approximate range 4.0 to 5.5.

3. The process of chemically plating with nickel a body essentially formed of a material selected from the group consisting of aluminum and its alloys, which comprises contacting said body with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate and also containing a fluoride, wherein the absolute concentration of nitrate anions in said bath expressed in mole/liter is between 0.15 and 0.50, and wherein the absolute concentration of fluoride anions in sai bath expressed in mole/liter is between 0.01 and 4. The process set forth in claim 3, wherein the initial pH of said bath is in the approximate range 4.0 to 5.5.

5. The process of chemically plating with nickel a body essentially formed of a material selected from the group consisting of aluminum and its alloys, which comprises contacting said body with a bath consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate and also containing a fluoride and a succinate, wherein the absolute concentration of nitrate anions in said bath expressed in mole/liter is between 0.15 and 0.50, and wherein the absolute concentration of fluoride anions in said bath expressed in mole/liter is between 0.01 and 0.04.

6. The process set forth in claim 5, wherein the ratio between nickel cations and hypophosphite anions in said bath expressed in molar concentrations is between 0.25 and 0.60, wherein the absolute concentration of hypophosphite anions in said bath expressed in mole/liter is between 0.15 and 0.30, wherein the absolute concentration of succinate anions in said bath expressed in mole/liter is at least 0.05, and wherein the initial pH of said bath is in the approximate range 4.0 to 5 .5.

7. A bath for the chemical nickel plating of aluminum and its alloys consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate, wherein the absolute concentration of nitrate anions in said bath expressed in mole/liter is between 0.15 and 0 0 8. A bath for the chemical nickel plating of aluminum and its alloys consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate and also containing a fluoride, wherein the absolute concentration of nitrate anions in said bath expressed in mole/liter is between 0.15 and 0.50, and wherein the absolute concentration of fluoride anions in said bath expressed in mole/liter is between 0.01 and 0.04.

9. A bath for the chemical nickel plating of aluminum and its alloys consisting essentially of an aqueous solution of a nickel salt and a hypophosphite and a nitrate and also containing a fiuoride and a succinate, wherein the absolute concentration of nitrate anions in said bath expressed in mole/llter is 'between0.15 and 0.50, and 1 wherein the absolute concentration of fluoride anions in gags bath expressed in mole/liter is between 0.01 and 10. The process of chemically plating with nickel a body essentially formed of a material selected from the group consisting of aluminum and its alloys, which comprises thoroughly degreasing and cleaning said body, then pickling said body in hydrochloric acid, rinsing said body in hot water, then immersingsaid body in an aqueous solution of nitric acid and hydrofluoric acid, again rinsing said body in hot water, and finally immersing said body in a bath consisting essentially of an 15' aqueous solution of a nickel salt and a hypophosphite and a nitrate, wherein the absolute concentrat1on of mtrate anions in said bath expressed in mole/liter is between 0.15 and 0.50.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,045,718 Marino Nov. 26, 1912 1,614,684 Pacz Jan. 18, 1927 2,217,921 Saukaitic Oct. 15, 1940 2,296,884 Thompson Sept. 29, 1942 2,532,283 Brenner Dec. 5, 1950 2,580,773 Heiman Jan. 1, 1952 

1. THE PROCESS OF CHEMICALLY PLATING WITH NICKEL A BODY ESSENTIALLY FORMED AT A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ITS ALLOYS, WHICH COMPRISES CONTACTING SAID BODY WITH A BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF A NICKEL SALT AND A HYPOPHOSPHITE AND A NITRATE, WHEREIN THE ABSOLUTE CONCENTRATION OF NITRATE ANIONS IN SAID BATH EXPRESSED IN MOLE/LITER IS BETWEEN 0.15 AND 0.50. 